In mobile communication networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the mobile communication network is deployed.
The evolution of the Universal Mobile Telecommunications Standard Long Term Evolution (UMTS LTE) for mobile communications continues with new features to increase the overall capacity and to increase the general performance of the communications network.
Wireless communication devices (hereinafter simply denoted wireless device, or WD for short) are typically built up in a modular fashion. Inside the WD there is typically one module/chipset supporting cellular systems, for instance GSM/WCDMA/LTE, and another separate chipset supporting short range wireless communications like WLAN and Bluetooth. Mainly due to cost reasons the cellular chipset in the WD is arranged only to support one active cellular Radio Access Technology (RAT), such as either GSM or WCDMA or LTE at the time. However the WD is typically arranged to handle one cellular RAT and a WLAN based RAT simultaneously.
In case there is cellular coverage, LTE would be the preferred RAT to use by the WD since LTE provides the highest data rates and lowest latency, etc. However LTE only supports packet switched (PS) services and does not support circuit switched (CS) services like speech in GSM and WCDMA. For voice services, voice over LTE (VoLTE) is a future technology to be used in LTE, but it will take a long time until an internet protocol (IP) like voice service is in place everywhere and then LTE devices that are supporting CS voice services need to utilize CS fallback. Next follows a short description of CS fallback.
CS Fallback in LTE
LTE will not support CS voice services, as is used in WCDMA/UMTS and GSM. Furthermore, VoLTE services are currently not well supported, and will probably only gradually be introduced. Therefore, there has been, and still is, a need to introduce CS fallback solutions making it possible for LTE devices to be able to use speech services. Below follows a short description of how such fallback works in the case that a voice call either is initialed by the network (hereinafter denoted WD Terminated) or by the wireless device itself (hereinafter denoted WD Originated). FIG. 1 schematically illustrates a communication network illustrating the network nodes and interfaces used for CS fallback. These nodes will be further disclosed below.
Incoming Calls; WD Terminated Calls
The WD is arranged to register at the LTE network E-UTRAN with information about both PS and CS support (using WCDMA (3G) and/or GSM (2G)), and is assumed to be opera lively connected to an eNB of a serving cell in the E-UTRAN. When an incoming call for the subscriber arrives at the MSG server, a request is made to the Home Subscriber Service (HSS) for the location of the subscriber. The HSS then returns the information to an G-MSC that the subscriber is currently served by the SGs MSG. The call is then forwarded to the SGs MSG. The SGs MSC will then send a paging message over the SGs interface to a mobility management entity (MME) which will in turn inform the WD and require the WD to leave the LTE network (E-UTRAN) to accept the incoming voice call in a 2G or 3G cell. The WD is then arranged to perform the necessary instructions and start communication in the UTRAN or GERAN.
Switching from one radio access technology to another can be performed in several ways. In a redirect scenario the network is arranged to provide the WD with an instruction to select a different radio network. The instruction can contain information about the target cells to reduce the time it takes the WD to find a suitable cell and to establish communication with a network node in the cell. In another scenario a full Inter-Radio Access Technology (IRAT) packed switched domain handover from LTE to UMTS or GSM which is prepared in the network is performed and thus the interruption time is lower. In this scenario, the network can be arranged to instruct the WD to perform radio measurements. The results of those measurements are then used by the network to select a suitable target cell and to give the WD precise instructions of how to quickly connect to this cell to minimize the handover time.
Outgoing Calls; WD Originated Calls
When the user initiates a call (i.e., a so-called WD originated call), the WD is arranged to contact the network by transmitting an Extended Service Request message which contains a CS fallback indicator. The network is then arranged to decide, based on its capabilities and the capabilities of the WD, to perform one of the below actions:                A packet switched handover to a GSM or UMTS cell, which is the fastest way to associate the WD with a radio access technology by means of which the circuit switched call can be initiated.        A radio resource control (RRC) release with redirect to GSM or UMTS, optionally with information about possible target cells to decrease the time necessary to find the cell. This process may generally be somewhat slower than a handover as the WD is required to by itself (i.e. without relying on signalling of the LTE network) re-establish contact to the UMTS network.        An inter-RAT cell change order sent to the GSM network. Optionally, the network can include information on potential GSM cells in the area (a so-called Network Assisted Cell Change, NACC).        
Contacting the network prior to leaving the LTE network is necessary in order for the WD's context in the LTE base station (eNodeB) to be deleted and in order to acquire additional information on potential target cells which may speed up the process.
There are also other situations when the WD needs to perform a HO or reselection from LTE to GSM/EDGE or WCDMA/HS. For instance, the network node can, due to load balancing between different RATs, order a HO of a WD from LTE to HS or EDGE. Another situation could be that the WD is moved beyond coverage from an LTE cell and therefore requires triggering of a HO/reselection to 2G/3G systems.
As noted above, a typical WD is capable of camping on either a cellular system (LTE/HS/EDGE) or a WLAN for PS services. The decision to use a cellular RAT or a WLAN RAT could be performed in several ways. For example, the choice for the WD to camp on (or operatively connect to) LTE or WLAN may be based on a metric determined for the WD to camp on (or operatively connect to) a network node of the LTE and a network node of the WLAN. The metric may, for instance, be signal strength based. However, none of the above disclosed selection techniques take into consideration the case when the WD is requested to perform a HO from LTE to another other cellular RAT, due to a reason as disclosed above and how to determine which RAT that then should handle the PS services.
Hence, there is still a need for an improved handling of associating a WD with a RAT.